1. Field of the Invention
The present invention relates generally to circulating fluidized bed boilers and, more particularly to systems and methods of operating circulating fluidized bed boilers to improved reactant utilization.
2. Description of the Related Art
The combustion of sulfur-containing carbonaceous compounds, especially coal, results in a combustion product gas containing unacceptably high levels of sulfur dioxide. Sulfur dioxide is a colorless gas, which is moderately soluble in water and aqueous liquids. It is formed primarily during the combustion of sulfur-containing fuel or waste. Once released to the atmosphere, sulfur dioxide reacts slowly to form sulfuric acid (H2SO4), inorganic sulfate compounds, and organic sulfate compounds. Atmospheric SO2 or H2SO4 results in undesirable “acid rain.”
According to the U.S. Environmental Protection Agency, acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures. Prior to falling to the earth, SO2 and NOx gases and their particulate matter derivatives, sulfates and nitrates, also contribute to visibility degradation and harm public health.
Air pollution control systems for sulfur dioxide removal generally rely on neutralization of the absorbed sulfur dioxide to an inorganic salt by alkali to prevent the sulfur from being emitted into the environment. The alkali for the reaction most frequently used include either calcitic or dolomitic limestone, slurry or dry quick and hydrated lime, and commercial and byproducts from Theodoric lime and trona magnesium hydroxide. The SO2, once absorbed by limestone, is captured in the existing particle capture equipment such as an electrostatic precipitator or baghouse.
Circulating fluidized bed boilers (CFB) utilize a fluidized bed of coal ash and limestone or similar alkali to reduce SO2 emissions. The bed may include other added particulate such as sand or refractory. Circulating fluidized bed boilers are effective at reducing SO2 and NOx emissions. A 92% reduction in SO2 emissions is typical, but can be as high as 98%. The molar ratio of Ca/S needed to achieve this reduction is designed to be approximately 2.2, which is 2.2 times the stoichiometric ratio of the reaction of calcium with sulfur. However, due to inefficient mixing, the Ca/S molar ratio often increases to 3.0 or more to achieve desired levels of SO2 capture. The higher ratio of Ca/S requires more limestone to be utilized in the process, thereby increasing operating costs. Additionally, inefficient mixing results in the formation of combustion “hotspots” that promote the formation of NOx.
Thus, there exists a need for circulating fluidized bed boiler having improved reactant utilization for reduction of undesirable combustion products, which at the same time may also reduce NOx formation.